Roundup is sort of a nerve poison (or harmful enzyme) for plants whose active ingredient isn't known to affect other types of life. Not that this makes it 100% safe, just that it lessens the chances that it will cause problems to mammals or birds. Sort of like how BT is a disease only affecting caterpillars - should be totally safe for everything else but people always worry.
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No, toxic isn't like being pregnant. "Toxic" is an adjective describing the amount necessary to cause harm. There are toxic levels of just about everything, including things we need to survive. So "toxic is toxic - always" is not true. Salt is necessary, but too much is toxic.